Secondary electron emitter and method of making it



Aug. 29, 1939. w. J. H. SCHREINEMACHERS 2,171,227

SECONDARY ELECTRON EMITTER AND METHOD OF MAKING IT Filed July 2'7, 19381 UNI/EA TED ELECTRODE 0F Sl/EE T METAL BUR/V MAGNESIUM 2 IN Al? 70PRODUCE CLOUD 0F H07 PARTICLES 0F MAGNESIUM OXIDE HOLD COLD ELECTRODE IN(Z0110 ABOVE BURNING MAGNESIUM 3 (INT/l SEMI-TRAMSPARENTLAYER 0FMJGNES/UM OXIDE ['ONDENSES' 0N EL EZ'TRODE SURFACE INVENTOR. WI EMJ.I'I.SCHREIIVEMACHERS arm;

ATTORNEY.

Patented Aug. 29, 1939 PATENT OFFICE SECONDARY ELECTRON EMITTEB ANDMETHOD OF MAKING I'l.

Willem J. H. Schreinemachers,

Elndhoven,

Netherlands, assignor to N. V. Philips Gloeilampenfabrieken, Eindhoven,

Netherlands, a

corporation of Netherlands Application July 27,1938, Serial No. 221,565

In the Netherlands August 9, 1937 3 Claims.

lviy invention relates to a method of manufacturing secondary electronemitting electrodes and, to improved secondary electron emitters made inaccordance with my method.

' Secondary electron emitters consisting of a sheet of metal with alayer of oxide of an active metal, such as alkaline earth metal ormagnesium, have been made usually in the evacuated vessel in which theyare to be used, the layer of oxide being, produced by vaporizing theactive metal in vacuum to form a film of active metal on the core bodyor electrode, after which the film of active metal is oxidized toproduce on the core body a comparatively smooth dense layer of i5 oxide.This procedure is rather difficult and I complicated, and producesemitters having a ratio of three or four secondary electrons per primaryat the voltages commonly used.

The object of my invention is to provide an improved secondary electronemitter which can be made without resorting to an evacuated vessel andcan be exposed to the air without damage so that the finished emittermay be easily mounted in a tube. Another object of my invention is toprovide a simple method 01 manufacturing such an emitter without the useof an evacuated vessel or of the arrangements commonly used forobtaining the active metal in the form of vapor.

In accordance with my invention a secondary electron emitter electrodehaving high electron emissivity is made by bringing into contact with acore body of metal, such as nickel, copper, or similar metal, a cloud ofoxide of an active metal, such as an alkaline earth metal or magnesium,

which is burned in an oxidizing atmosphere or gas, such as air, toproduce a cloud of oxide from which the oxide particles settle on andadhere to the core body to form a coating. By this method it is possibleto coat a metal electrode in a very simple manner with a material whichreadily emits secondary electrons without any need of utilizingcomplicated processes which must be carried out within an evacuatedvessel.

In the accompanying'drawing Figure 1 shows 5 steps to be taken inaccordance with the invention, and Figure 2 shows in greatly enlargedcross-section a portion of an electrode made in accordance with theinvention.

In Figure 1 the rectangles numbered I, 2, and

50 3 represent successive steps by which an electrode can be produced inaccordance with my invention.

Figure 2 shows the finished electrode comprising a metal core body 4,preferably of sheet nickel,

on the surface of which is a thin semi-trans- E par n layer 5 a g um fme or oxide condensed on the surface of the electrode from the smoke orfumes of magnesium burning in air, and preferably while the electrode iscolder than the particles of oxide.

The cloud of oxide of the active metal is most 5 conveniently producedby burning a metal, such as magnesium, either in air or an oxidizinggas, in the vicinity of the metal core body so that the core body isexposed to the cloud of oxide under such conditions and for such alength of time 10 that a magnesium fume layer of the oxide from thecloud will form on the surface of the core body. The metal core body ispreferably near room temperature and the burning magnesium or otheractive metal is preferably so placed 15 in relatlon'to the core bodythat the oxide forming the cloud to which the core body is exposed ishotter than the core body so that hot particles of oxide from thecloud'deposit on the cold metal core body. In this way a somewhat moread- 20 herent coating is obtained than when the cloud of oxide and thecore body are at substantially the same temperature. The coating mayconveniently be done by burning a strip of magnesium in air while thecore body, for .example, of 25 sheet nickel, is held above the burningmagnesium and in the cloud of smoke or oxide. It is advantageous to burnthe magnesium in a gas flame above which the sheet nickel electrode isheld, so the smoke of the burning magnesium is 30 carried to anddeposited on the nickel sheet. The layer of oxide formed from the cloudon the core body should be thin enough to be semi-trans- .parent so asto permit the nickel to be visible through the layer. The surface of thecore body 35 may have a slightly dull or flufly appearance, which isprobably due to a fluify porous coating of oxide. The secondary electronemitting electrode produced in this way can be handled in the open airand can be mounted without any 0 special precautions and has a highercoeflicient of secondary electron emission than oxide coated electrodesproduced in the conventional way. For example, the ratio of secondaryelectrons to primary electrons may be as high as six to one or moredependent on the thickness of the layer as compared to about four to oneobtained at the same voltage from the usual emitters coated withmagnesium oxide. It is probable that the difference in secondaryelectron emissivity arises from the difference in physicalcharacteristics between the rough, porous coating produced by depositingvery fine particles of oxide, such as magnesium oxide, from a cloudobtained by buming the magnesium and the comparatively smooth densecoating of magnesium oxide obtained in other'ways such as by oxidizing afilm of magnesium on a metal core body. The chemical composition of themagnesium oxide layer on my improved electrode seems to be substantiallythe same as that of the magnesium oxide layer on electrodes heretoforeknown, but the physical characteristics are quite difierent and, as aresult, the secondary electron characteristics are markedly diii'erentand better than those of the magnesium oxide coated electrodesheretofore known.

A secondary emitting electrode made in accordance with my invention maybe used to advantage in electron amplifiers of the conventional type andwith marked advantage as secondary electron emitters in secondaryemission amplifying tubes such as disclosed in United States patents toVan Overbeek No. 2,146,607, issued February 7, 1939, and to Lopp et al.No. 2,151,783, issued March 28, 1939.

t I claim: v

1. The method of making secondary electron emitting electrodes havinghigh secondary electron emissivity, which comprises producing a cloud ofoxide of an active metal selected from the alkaline earth metals andmagnesium by burning the active metal in an oxidizing atmosphere, andexposing a metal core body to said cloud of oxide to form on said corebody a layer of oxide from said cloud.

2. The method of making a secondary electron emitting electrode havinghigh secondary electron emissivity, which comprises producing a hotcloud of fine particles of magnesium oxide by burning magnesium in air,and exposing to said hot cloud of oxide particles a cold sheet of nickelto condense from said cloud on said sheet a rough fluffy porous layercomposed of very fine particles of oxide.

3. A secondary electron emitting electrode comprising a sheet metal corebody, and a fluffy, porous, semi-transparent layer of very finemagnesium oxide on the surface of said core body.

W. J. H. SC HREINEMACHERS.

